Refraction Artefacts
Refraction occurs when the ultrasound beam crosses a boundary between tissues that have different speeds of sound propagation. The sound beam ‘bends’ from its original path.
The appearances generated within the image can be confusing. A classic example of this is the ‘disappearing twin’ artefact.
Disappearing twin artefact
This image is a transverse image of the pelvis demonstrating the urinary bladder, the uterus and both ovaries (yellow arrows). The uterus appears to contain two adjacent gestation sacs (green arrows) though only one is present.
Ultrasound equipment is programmed to assume that sound travels in straight lines, and, that received echoes have come from a source directly in line with the transmitted sound pulse. This is not always a correct assumption.
Echoes generated from the refracted beam, as it continues its journey beyond the boundary will therefore be misplaced in the image. The equipment will assume that these echoes originated along the original beam pathway echoes are therefore picked up (and displayed in the images) from off-axis structures.
By scanning from a more lateral/oblique approach, the degree of refraction will be reduced and the artefact will disappear.
A similar refraction artefact appearance can sometimes be generated in the upper abdomen, where duplicated abdominal vessels are seen. This is also a refraction artefact.
Duplicate abdominal vessels
In this image:
- Duplicated Superior mesenteric artery (SMA) (green arrows)
- Duplicated aorta (yellow arrows)
Note, the clue is the slightly dumbbell shape of the vertebral body.